Cellulose is an abundant, renewable, low cost bio-material. However, the poor solubility of cellulose in common organic solvents significantly limits its utilization in many applications because of processing difficulties. However, recently several studies have reported the chemical modification of cellulose (e.g. acylation, carbanilation, etc.), dissolved in solutions of ionic solvents.
Cellulose derivatives such as cellulose acetate, cellulose diacetate and cellulose triacetate have been used to prepare commercial reverse osmosis membranes by phase inversion methods. However, such membranes have various problems attributable to the membrane structure. For example, in order to provide commercially useful filtration rates, high operating pressures (up to 1 MPa) must be used, which increases energy costs and results in a loss of separation performance and mechanical breakage of the membrane due to compaction and densification of the membrane during the filtration process. Membranes with thinner dense layers have been prepared in order to increase the flux rate, but such membranes are prone to breakage even at low pressure.
Compared to cellulose derivatives such as cellulose acetate, cellulose membranes offer improved properties because of their excellent chemical stability, biocompatibility, and environmental friendliness. Cellulose ultrafiltration (UF) membranes have been prepared e.g. by hydrolyzing conventional phase inversion cellulose acetate membranes in strong basic solutions such as aqueous sodium hydroxide. This process is relatively complex and expensive, and the organic solvents (e.g. NMP) used to prepare the phase inversion membrane and the corrosive base solutions are not environmentally friendly and are toxic and/or hazardous to use.
Most commercial reverse osmosis (RO) membranes currently used for desalination are composite membranes made by an interfacial polymerization process. Typically, a microporous membrane (e.g., a polysulfone UF membrane) is first soaked in an amine solution. The aromatic amine-wetted UF membrane support is then contacted with one or more crosslinking agents dissolved in an immiscible organic solvent(s) (e.g., trimesoyl chloride in hexane). At the interface of the two immiscible liquids, a dense, crosslinked, and charged polymeric network is formed. Such interfacially polymerized top coating layers typically have a thickness of ˜0.002 to ˜0.3 μm. Current commercial RO membranes have the sodium chloride rejection rate of 99+% and a water flux greater than 35 L/m2h at a feed pressure of 800 psi.
The majority of commercially available nanofiltration (NF) membranes are also prepared by interfacial polymerization, e.g., comprising a piperazineamide on a microporous substrate. For example, Cadotte et al. (U.S. Pat. No. 4,259,183, herein incorporated by reference in its entirety for all purposes) has successfully demonstrated the fabrication of NF membranes by the interfacial polymerization of piperazine using trimesoyl chloride. These composite nanofiltration membranes exhibited very high MgSO4 rejection rate (99%) but low NaCl retention rate (<60%). Multi-component (piperazine and polyvinyl alcohol, JP 61 93,806; herein incorporated by reference in its entirety for all purposes) and multi-layer coating (sulfonated polysulfone and piperazineamide) composite membranes have also been prepared. For typical nanofiltration membranes, the molecular weight cutoff ranges are from 100 to 5000 Dalton, with a high rejection of divalent ions (>99%) and low rejection of monovalent ions (˜50% or less).
Composite UF membranes have also been prepared by interfacial polymerization. Wrasidlo et al (U.S. Pat. No. 4,902,424, herein incorporated by reference in its entirety for all purposes) prepared composite UF membranes by the interfacial polymerization of a polyethyleneimine-soaked microporous membrane with isophthaloyl chloride and toluene diisocyanate in hexane. The polymerized top coating layer had a thickness ranging from 0.0012 to 0.15 μm, with molecular weight cutoff values ranging from 500 to 1,000,000 Dalton. Stengaard et al (J. Membr. Sci., 53 (1990) 189-202; herein incorporated by reference in its entirety for all purposes) reported reacting an undisclosed aqueous monomer composition with diisocyanates on polyethersulfone UF membranes (MWCO: 20 k˜50 k Dalton). Separation of whey/skimmed milk mixtures were carried out, with a permeate flux ranging from 40˜75 L/m2h at 30˜60 psi.
However, a major drawback in conventional composite membranes prepared by interfacial polymerization processes is pore blockage in the microporous membrane support when it is soaked in aqueous amine solutions. The blocked pores tend to increase the effective coating thickness of the interfacially polymerized coating layer, and consequently tend to decrease the permeate flux. Also, the chemical nature of polyamide coating (e.g. hydrolyzed acyl halide; carboxylate groups and terminating amine groups), make interfacially polyamide composite membranes more prone to fouling by charged solute species, which also tends to significantly reduce the permeate flux. Typically, before the use of interfacially polymerized polyamide coated membrane in the final step of filtration (NF and RO), the feed solution must be pre-filtered by microfiltration and ultrafiltration in order to keep a stable flux rate without significant fouling. Another drawback in the preparation of the conventional composite membranes is the use of volatile organic solvents and corrosive base solutions for the hydrolysis of cellulose esters in organic solvents.
Thus, there is a need for high-flux UF, NF, forward osmosis (FO) and RO membranes having a high permeation rate, high rejection ratio, reduced fouling rate prepared using an environmentally benign process compared to filtration membranes currently available on the market today. The cellulose composite membranes prepared by the process of the present invention provide improved properties and are easily prepared using environmentally friendly solvents.